The present invention relates to an heat roll fixing device employed in an electrophotographic imaging apparatus.
Conventionally, an electrophotographic imaging apparatus, such as a copy machine, laser beam printer and the like, is known wherein the uniformly charged surface of a photoconductive drum is exposed to light to form a latent image, charged toner is adhered to the latent image and developed to a toner image, and the toner image is then transferred onto a recording sheet and fixed thereon by a fixing device.
The electrophotographic imaging apparatus generally uses a so-called heat roll fixing device which heats, and pressingly fuses the toner image transferred on the recording sheet with the heat applied from a heated roller (heat roller) then fixes the same on the recording sheet. The heat roll fixing unit comprises a pair of fixing rollers composed of a heat roller and a press roller (also referred to as a back-up roller). The heat roller is composed of a cylindrical roller having a heating element, such as a halogen lamp or the like, inserted therein and heated to a predetermined temperature. The back-up roller is formed of a material, such as silicone rubber, having a heat resistant property and a predetermined elasticity and pressed against the heat roller by a predetermined pressure. The recording sheet carrying an unfixed toner image is caused to pass between the pair of fixing rollers to make the toner image to be heated and pressed so that the toner is fused and fixed on the recording sheet. This kind of the heat roll fixing device is advantageous in that it has an excellent heat efficiency and the fixing operation can be performed at a high speed.
The heat roller is controlled by feedback and thus kept at a temperature suitable for a fixing operation at all times. Usually, the heat roller is driven to rotate and then the back-up roller is rotated by the rotation of the heat roller; thus a recording sheet held between the pair of fixing rolls is fed by the fixing device (i.e., the pair of fixing rollers).
Nevertheless, the fixing device described above has a drawback in that the temperature of the back-up roller is irregularly distributed depending upon the width of a recording sheet to be subjected to a fixing operation. Thus, the back-up roller is irregularly expanded and deformed by the irregular temperature distribution, so that the fixing property and recording sheet feed capability of the fixing rollers are deteriorated.
More specifically, as shown in FIG. 1A, the temperature of the portion of the fixing device where a fixing operation is carried out (the portion contacting a recording sheet 20) is lowered because the heat of the portion is absorbed by the recording sheet 20 and the heat of the portion thereof where the fixing operation is not carried out is not absorbed by the recording sheet 20, and thus the temperature of a heat roller 11 in the axial direction thereof is irregularly distributed. Since an elastic material constituting a back-up roller generally has a large coefficient of thermal expansion, and since the heat of the back-up roller 12 in the axial direction thereof is also irregularly distributed in correspondence to the above irregular temperature distribution of the heat roller 11, the outside diameter of the portion in the axial direction thereof where the fixing operation is not carried out (the portion where the recording sheet does not contact) is enlarged and as shown in FIG. 1B, only the portion of the back-up roller where the recording sheet does not contact is pressed by the heat roller 11. (Note that any one of the heat roller 11 and the back-up roller 12 can be moved in the direction perpendicular to the axial direction thereof and urged toward another roller by an urging member, such as a spring, to provide a predetermined pressure and also note that FIG. 1B is exaggeratively drawn). As a result, a pressing force applied to the portion which contribute to the fixing operation is lowered and thus the fixing property and recording sheet feed capability are deteriorated. This problem arises more often when using a recording sheet having a width that is considerably narrower than the maximum width of a recording sheet which can be fixed with the fixing device (in other words, when a recording sheet having the width far narrower than the length of the heat roller is subjected to a fixing operation).
Further, the surface of the heat roller is finished to have a lower friction coefficient in order to prevent the adhesion of toner, and thus the feed of the recording sheet solely depends upon the back-up roller. With the arrangement that the back-up roller is rotated by the rotation of the heat roller, however, if the outside diameter of the portion of the back-up roller where the recording sheet contacts (the portion which contributes to the fixing operation of the loaded recording sheet) is made relatively smaller than that of the portion where the recording sheet does not contact the recording sheet, the circumferential speed of the back-up roller becomes smaller than a set value, thereby the feed speed of the recording sheet is lowered. As a result, if a recording sheet is a continuous paper or the like and there is provide a recording sheet feed mechanism in addition to the heat roll fixing device, the recording sheet feed speed thereof is not synchronized between the additional feed mechanism and the fixing unit, and the recording sheet may become loosened or jammed.
Further, since the heat roller has an irregular temperature distribution in the axial direction thereof which is caused by the width of a recording sheet, a temperature sensor for controlling the heat roller by feedback is preferably disposed to measure the temperature of the portion of the heat roller where the fixing operation is carried out. Even if, however, the temperature of the fixing portion is correctly measured, the temperature of the portion of the heat roller where the recording sheet does not contact (the portion through which the recording sheet does not pass) continuously increases to a high temperature and thus may adversely affect peripheral components and units.